A phase-locked loop (PLL) is a closed-loop feedback control system that maintains a generated signal in a fixed phase relationship to a reference signal. More importantly, the PLL is used widely in radio, telecommunications, computers and other electronic applications where it is desired to stabilize a generated signal or to detect signals in the presence of noise. For example, the PLL is widely used for synchronization purposes, in communication for coherent carrier tracking, bit synchronization and symbol synchronization for instance.
FIG. 1 shows a fast lock circuit 100 for a PLL circuit. The fast lock circuit 100 uses the normal PLL phase frequency detector 110 and normal PLL charge pump 130 combination in the primary loop. Before the PLL circuit has achieved lock, a separate frequency detector 120 and a secondary charge pump 140 quickly charge or discharge the large loop filter capacitor 150.
Moreover, when the PLL approaches lock, a deliberate dead zone built into the frequency detector 120 disables the secondary circuit. As a result, the phase frequency detector 110 and the charge pump 130 bring the PLL to lock. However, as the PLL approaches lock, the phase frequency detector produces a pump down signal on every period of the reference clock. The pump down signal resets the pump up signal from the phase frequency detector which slows down the net charging of the loop filter as shown in FIG. 2. As a result, the lock time increases as shown by the discontinuity of the filter charging curve of FIG. 2.
An H-bridge circuit is commonly used in charge pumps in order to maintain a constant voltage across both the current source and the current sink in the charge pump of a PLL. However, when an H-bridge is used and both phase frequency detector pulses have the same value, the current is shunted to ground. As a result, during the time that the current is not steered onto the loop filter, the current is shunted to ground and wasted because it is no longer used to charge the loop filter. As a result, the lock time increases.